This invention relates to a device for electrochemically measuring the concentration of oxygen in combustion gases which uses an ion-conducting solid electrolyte and more particularly to an improvement in the construction of an ion-conducting solid electrolyte member for such device.
It is known that an ion-conducting solid electrolyte member produced by firing zirconia (ZrO.sub.2), for example, can have application in a device for electrochemically measuring the concentration of oxygen in combustion gases.
A device of the type described which uses an ion-conducting solid electrolyte member is shown in U.S. Pat. No. 3,960,693, for example.
Heretofore, it has been common practice to produce an ion-conducting solid electrolyte member by forming zirconia or other raw material, as by rubber press forming, into a sack-like tubular shape which is successively subjected to baking, machining and firing. The ion-conducting solid electrolyte member produced in this way is formed with electrodes on its outer and inner surfaces.
In producing an ion-conducting solid electrolyte member by the aforementioned method of the prior art, it is impossible to produce an element of the sack-like tubular shape having a wall thickness of high precision merely by subjecting zirconia or other material to rubber press forming and machining the element produced in this way.
Highly advanced skills are required for forming electrodes of a uniform thickness on the inner and outer surfaces of the element by means of a brush or spraying.
When electrodes are formed by using the techniques of forming a thin layer in vacuum including sputtering, ion plating and vaporization deposition, it is necessary to form the electrodes on the inner and outer surfaces of the element in separate operations. Since the element is of the sack-like tubular shape, the electrode formed on the outer surface of the element tends to have a greater thickness in portions thereof which are nearer to the forward end of the element of the sack-like tubular shape. Conversely, the electrode formed on the inner surface of the element tends to have a greater thickness in portions thereof which are nearer to the flange of the element of the sack-like tubular shape.
Thus a device of the prior art for electrochemically measuring the concentration of oxygen in combustion gases has had the disadvantage that if the formation of electrodes in the ion-conducting solid electrolyte member is not carried out satisfactorily it is impossible to take out an output signal from such member which represents an actual value of the concentration of oxygen.